High dose interleukin-2 (IL-2) therapy has been shown to be effective against human melanomas. IL-2 has also been used to overcome immunosuppression in patients treated with chemotherapy and to treat viral infections including AIDS so as to facilitate recovery of CD4+ T cells. However, such a therapy is also accompanied by severe life threatening toxicity characterized by vascular leak syndrome (VLS), which results from endothelial cell (EC) injury. EC damage has also been noted in certain infections, experimental allergic encephalomyelitis, vasculitides, rheumatoid arthritis, giant cell arthridis as well as following transplantation and graft-versus-host disease. We have demonstrated that IL-2 administration leads to upregulation of several CD44 isoforms on cytotoxic lymphocytes that migrate to various organs and cause EC damage by using CD44 as an effector molecule. We have shown that CD44KO mice are more resistant to IL-2 induced VLS and furthermore, VLS can be effectively blocked by CD44 antagonists. In the current study we will test the hypothesis that preventing EC injury would permit the use of high doses of IL-2 to effectively treat melanomas. To address this, we will 1) Identify the CD44 isoforms used by cytolytic cells, EC and tumor cells to delineate their role in EC injury and tumor metastasis 2) Use mice deficient in all isoforms of CD44 or those deficient in V7 or V6 and V7 exons to test whether decreased EC injury would facilitate immunotherapy of melanomas. 3) Use CD44 antagonists such as hyaluronic acid (HA), Abs to CD44 isoforms, CD44:Fc fusion proteins, etc. to prevent VLS, block metastasis of melanoma and facilitate tumor therapy. 4) Develop a fusion protein consisting of a part of IL-2 and antigen binding sites of anti-CD44. We will use this protein to prevent tumor metastasis and increase the efficacy of IL-2 in the treatment of melanomas, and 5) Study the differential effects of IL-2 on cytotoxic T cells and natural killer cells by using IL-2 mutein and NK cell deficient NKCD Tg mice. Together, the current study should provide novel insights into the varied function of CD44 isoforms and how these molecules can be manipulated to enhance IL-2 based melanoma therapy. The proposed studies also form the basis for preventing EC injury in wide range of clinical manifestations.